The present invention provides a system for converting the format of a digital video bitstream. The invention is particularly suitable for converting a MPEG-2 digital video data from a 4:2:2 Profile format to a Main Profile format.
The following acronyms and abbreviations are used:
4:2:2P--4:2:2 Profile; PA1 CBP--Coded Block Pattern; PA1 DCT--Discrete Cosine Transform; PA1 HDTV--High Definition Television; PA1 HL--High Level; PA1 IDCT--Inverse DCT; PA1 MB--Macroblock PA1 MC--Motion Compensation; PA1 ML--Main Level; PA1 MP--Main Profile; PA1 MV--Motion Vector; PA1 QDC--Quantized Direct Current; PA1 SDTV--Standard Definition Television; PA1 VBV--Video Buffer Verifier; PA1 VLC--Variable Length Coder; and PA1 VLD--Variable Length Decoder. PA1 Storage PA1 Editing and creation of visual effects PA1 Video tape or disk recording for professional use (contribution quality) PA1 Studio post-production of high-quality video sequences PA1 Efficient transmission for storage and distribution of contribution quality video
A video image is defined by a number of picture elements, also known as pixels or pels. A pixel, which is the smallest element of a raster scan line in the image, has an associated color space. For example, in a YCrCb color space, Y is a luminance component, and Cr and Cb are color difference components. Various sampling formats have been defined, including 4:4:4, 4:2:2, and 4:2:0. For example, with a 4:2:2 format, a macroblock has four 8.times.8 Y blocks, two 8.times.8 Cr blocks and two 8.times.8 Cb blocks. With this format, the sampling frequencies for the Y, Cr and Cb components may be 13.5 MHz, 6.75 MHz and 6.75 MHz, respectively.
With a 4:2:0 format, a macroblock has four 8.times.8 Y blocks, one 8.times.8 Cr block and one 8.times.8 Cb block.
Moreover, various digital video coding standards have been developed for coding video data including, in particular, the MPEG-2 standard, defined in ISO/IEC 13818-2 MPEG-2 Video (ITU-R H.262), 1995.
MPEG-2 designates several sets of constrained parameters using a two-dimensional ranking order. One of the dimensions, called the "profile" series, specifies the coding features supported. The other dimension, called "level", specifies the picture resolutions, bit rates, and so forth. that can be accommodated. The most important profile-level combination is called Main Profile at Main Level, or MP@ML. MP@ML supports a 4:2:0 color subsampling ratio.
In addition to the Main Profile, other profiles exist. For example, the Simple Profile is similar to the Main Profile but has no B-pictures. The SNR Scaleable Profile adds SNR scalability to the Main Profile. The Spatially Scaleable Profile adds spatial scalability to the SNR Scaleable Profile. The High Profile adds a 4:2:2 color format capability to the Spatially Scaleable Profile.
The Main Level is defined for CCIR 601 video, while the Simple Level is defined for Standard Intermediate Format (SIF) video. Additionally, two higher levels for HDTV are the High-1440 Level, with a maximum of 1440 pixels per line, and the High Level, with a maximum of 1920 pixels per line.
Furthermore, a 4:2:2 profile, also referred to as 4:2:2 P, has recently been developed, which accommodates both 4:2:2 and 4:2:0 color formats. See ISO/IEC 13818-2 Amendment 2, MPEG-2 4:2:2 Profile at Main Level, January 1996; ANSI/SMPTE 308 M, SMPTE STANDARD for television--MPEG-2 4:2:2 Profile at High Level, 1997; and ISO/IEC 13818-2 Proposed Draft Amendment 5, 4:2:2 Profile at High Level, March 1998, each of which is incorporated herein by reference.
The 4:2:2 profile is intended for professional video applications where ease of editing of compressed video and multiple-generation encoding/decoding of video are important requirements. The primary applications targeted by this profile are:
4:2:2 P can provide higher video quality, better chroma resolution and allows a higher bit-rate (at Main Level(ML), up to 50 Mbit/s) than Main Profile (e.g., MP@ML). In particular, 4:2:2 P provides separate quantization matrices for luma and chroma data. In studio applications, very high quality video and ITU-R 601 4:2:2 video format are often needed for ease of chroma keying and other special effects. Because of the requirement of ease of editing, more frequent INTRA pictures are necessary, which also results in high coding bit-rates. 4:2:2 P permits all I-picture encoding. This enables fast recovery from transmission errors and can simplify editing applications.
Even the 4:2:2 P with a 4:2:0 color format can provide a better quality image than MP with the same color format since 4:2:2 P allows greater quantization precision. 4:2:2 P further allows the high bit rates required to maintain high quality while using only I-picture coding. 4:2:2 P also allows the use of P- and B-picture coding types, which can further improve quality or reduce bit rate for the same quality.
Furthermore, in a typical TV broadcast chain, video goes through multiple generations of encode/decode as local TV stations add local programming information and commercials to video before it gets distributed to consumers for reception at their homes, e.g., via a cable television network. Moreover, with analog TV, multiple generations of encode/decode can result in significant picture quality losses. On the other hand, 4:2:2 P can preserve high quality after multiple generations of encoding/decoding. In the case of multiple generations without picture manipulation or change in picture coding type between generations, the quality from the 4:2:2 P coder remains nearly constant after the first generation. Use of picture manipulation or change in picture coding type between generations causes some degradation in quality. Nevertheless, the resulting quality is acceptable for a broad range of applications.
TV studios typically produce "contribution quality" video, which usually is ITU-R 601 source video, or 4:2:2 P@ML (or 4:2:2 P@HL) compressed video. This video is then encoded and transmitted as "distribution quality" TV. Digital television and HDTV in North America have adopted MPEG-2 MP@ML and MP@HL as their video coding standard. Both MP@ML and MP@HL encode and transmit only 4:2:0 format distribution quality video. MP provides a common quantization matrix for luma and chroma data. For example, the aggregate data rate for a HDTV system, which includes compressed video, compressed audio, conditional access, and an auxiliary data channel, is around 18 to 20 Mbits/s.
Accordingly, there is a need for an efficient system to convert a 4:2:2 P pre-compressed contribution quality bitstream to a MP distribution quality bitstream.
To reduce coding complexity, the system should allow reuse of motion vectors, avoid the need to change MB coding types, and use only a single MC unit for processing chroma blocks. The system should further provide the capability to perform a decimate-filtering process on residue chroma-blocks in the INTER coded MBs.
The system should provide a simple approach to replace the chroma quantization matrix during format conversion it if appears in the 4:2:2 P bitstreams.
The system should refine the CBP for chroma blocks.
The system should also provide adjustment of the MPEG DC coefficient precision variable, intra_dc_precision, as required to conform to the coding standard used. For example, the maximum allowed intra_dc_precision level for a MP bitstream is lower than that for a 4:2:2 P bitstream.
The system should provide a combined 4:2:2 to 4:2:0 converter and a normal transcoder to perform 4:2:2 P to MP transcoding with a format conversion.
The system should avoid the need to fully decode the processed bitstream during the format conversion.
The present invention provides a system having the above and other advantages.